This invention relates to photodetectors and more particularly to infrared-sensitive photodiodes.
It is well established that single crystal films of lead chalcogenides, lead tin chalcogenides, and lead cadmium chalcogenides can be epitaxially grown on heated alkali halide and alkaline earth halide substrates by vacuum evaporation. The chalcogenides used include the sulfides, selenides, tellurides, and mixtures thereof. The substrates are single crystals of infrared transparent alkali halides and alkaline earth halides. Examples include barium fluoride, strontium fluoride, calcium fluoride, lithium fluoride, sodium chloride, potassium chloride, etc.
It is also well known that the vacuum deposition of a metal contact of certain materials such as lead or indium, on the surface of an epitaxial layer of a lead chalcogenide, lead tin chalcogenide, or lead cadmium chalcogenide creates a non-Ohmic Schottky barrier at the point of contact, resulting in an infrared sensitive photodiode. Vacuum depositing a contact of certain other metals (e.g., Au, Ni, Pb, or Pt) at another point on the epitaxial layer provides the Ohmic contact necessary for measuring the photovoltage of the device.
Attention is called to U.S. Pat. No. 4,263,604, entitled "Graded Gap Semiconductor Detector," issued on Apr. 21, 1981 to James D. Jensen and Richard B. Schoolar wherein an extensive bibliography of articles and patents dealing with these Schottky barrier devices is listed in the background of the invention.
Despite the usefulness of these prior art devices and processes, there are two areas where improvement would be desirable. First, it would be desirable to improve the reliability of fabrication of these devices and second, it would be desirable to increase the performance of these devices.
A. C. Bouley, H. R. Riedl, J. D. Jensen, S. R. Jost in U.S. patent application Ser. No. 358,941, filed Mar. 17, 1982, entitled "Sensitized Epitaxial Infrared Detector," disclose that the presence of halide ions in the interface region between a non-Ohmic lead metal and an epitaxial layer of II-IV-VI semiconductor alloy material (i.e., lead chalcogenide, lead tin chalcogenide, or lead cadmium chalcogenide) increases the performance of these infrared detecting diodes. The number of rejects in the manufacturing process was also reduced.
Nevertheless, it would be desirable to have a still more reliable method of making these diodes.